What is the equivalent of correspondence today?
I guess back then each letter had a cost, in (delivery) time and money, so you better make it count.
My guess is that these correspondences were often interesting to read because they had to be worthwile to send because of the associated cost.
Time of delivery would be the biggest factor. Today we can send multiple quick messages to anyone, at that time people had to batch big discussions in a long articulated text, since it would take days to it arrive.
I guess the closest to that nowadays would be blog articles, RFC discussions or long-form email threads.